The present invention relates to magnesium-based alloys and more particularly to light structural alloys which can be used for manufacturing parts of various constructions when the most important requirements imposed on the items are their light weight, rigidity, and high strength at temperatures of up to 300.degree. C upon prolonged operation and up to 400.degree. C upon short-term service.
Known in the art are numerous magnesium-based alloys with various alloying additives, the choice of such additives being dependent on the requirements imposed on the alloy, since some additives ensure high heat-resistance and others high strength at room temperature.
However, as will be shown below in greater detail all industrial magnesium-based alloys known at present, not containing a radioactive additive of thorium, are applied, as a rule, at temperatures not exceeding 200.degree.-250.degree. C at best.
For example, known in the art are high-strength alloys AZ-92, AZ-91, and ZK-61 having tensile strength of 27-30 kg/mm.sup.2. But these alloys are not heat-resistant, since above 150.degree. C their mechanical properties deteriorate drastically. The alloy QE-22 is another heat-resistant alloy which, due to its good mechanical properties, can be used for manufacturing parts which operate at 200.degree. - 250.degree. C for a long period of time and at 300.degree. - 350.degree. C for short intervals.
Also known in the art is a magnesium-based alloy containing 0.1 - 10 wt. % yttrium, 0.1 - 10 wt. % zirconium, up to 1.25 wt. % zinc, 0.15 - 0.5 wt. % manganese, and up to 3.0 wt. % are rare-earths; the rest is magnesium. The alloy is described in Belgian Pat. No. 654,809. However, poor mechanical properties at 20.degree. C (yield strength 7.2 kg/mm.sup.2) and low heat-resistance limit the application of this alloy at temperatures up to 150.degree. C under conditions of prolonged load.
Another alloy, which is more heat-resistant, is described in U.S. Pat. No. 3,157,496. The alloy contains, in addition to magnesium, 0.05 - 0.5 wt. % zinc and 0.05 - 2.0 wt. % of a rare-earth element. However, creep limit of the alloy at 205.degree. C is 3.5 kg/mm.sup.2, which makes the alloy unsuitable for manufacturing parts operating at temperatures above 200.degree. C.
Various attempts were made to increase the yield point in compression by means of enhancing the zinc content. Thus, for example, in U.S. Pat. No. 3,183,083 a magnesium-based alloy is described which contains 7 - 16 wt. % zinc, 0.1-1.0 wt. % zirconium and 1 - 8 wt. % of a rare-earth element. In addition, a possibility is considered of introducing rare-earth metals into the alloy from a mischmetal.
It should also be noted that alloys with such a high zinc content have low heat-resistance. Likewise known in the art is an alloy for manufacturing pressed half-finished products (see U.S. Pat. No. 2,549,955), containing no less than 85 wt. % magnesium, 0.4 wt. % zirconium, and from 0.5 to 4.0 wt. % rare-earth metals, including neodymium, cerium and lanthanum. But it is known that alloys containing cerium and lanthanum have poor mechanical properties at room temperature and are intended for operation at temperatures up to 250.degree. C.
All of the above-cited alloys are intended for manufacturing articles by pressing, forging, stamping, and by other similar techniques and have relatively low heat-resistance. Therefore, such alloys did not find application for casting shaped parts operating at high temperatures.
Widely known are heat-resistant magnesium-based alloys containing thorium. For example, the alloy HK-31 contains in wt. %: from 2.5 to 4.0 thorium, from 0.4 to 1,0 zirconium, no less than 0.3 zinc, magnesium being the rest; the alloy HZ-32 contains in wt. %: from 2.5 to 4.0 thorium, from 0.5 to 1.0 zirconium, from 1.7 to 2.5 zinc, magnesium being the rest. However, the production of these alloys involves radiation hazards for the attending personnel because of the radiological toxicity of thorium, and calls for special protection.